1. Field of the Invention
The present invention relates to visual screening systems and methods, and more particularly, to a method and apparatus screening monocular visual acuity to detect vision disorders, such as amblyopia.
2. Background Information
It is recommended to screen children at an early age for vision disorders. The American Academy of Pediatrics, along with other medical professional organizations, recommend that children have their visual acuity quantified at least by age four. In many instances, it may be possible to correct a child's vision if problems are detected during early childhood.
Amblyopia is the leading cause of reversible blindness in children in the United States, affecting approximately 2-4% of the population. In order to successfully treat amblyopia, a child must be diagnosed with this condition at an early age. If the condition is detected sufficiently early, it is often possible to completely, or at least substantially, correct the child's vision. However, if left untreated by seven to nine years of age, it may only be possible to slightly improve a child's vision, if at all.
Amblyopia, also known as “lazy eye,” is a condition in which a patient's brain processes substantially more visual information from one eye than the other, such that the patient is only “seeing” with one eye. This typically develops during early childhood, when a child compensates for reduced vision in one eye by neuro-developmentally learning to see only through the other eye. Amblyopia may result from a misalignment of a child's eyes, known as “crossed eyes.” Amblyopia can also be caused by a marked difference in visual acuity between a child's eyes, causing the child to focus through only one eye. This is called anisometropic amblyopia. For more information about amblyopia, see    http://www.preventblindness.org/children/amblyopiaFAQ.html.
If detected at an early age, the debilitating effects of amblyopia may be avoided in many circumstances by training a child's visually immature brain to process images from the affected eye. The amblyopic eye is commonly treated by using a patch to cover a patient's “stronger” eye, which forces the patient to use the weaker eye. Another alternative method to “patching” is to use eye drops to “blur” the vision in a child's stronger eye. For a patient with anisometropic amblyopia, who is unable to see normally through one or both eyes due to an uncorrected refractive error, proper spectacle correction (glasses) is also prescribed to be worn full time, simultaneously with the “patching” or “blurring” treatment, to focus the blurred image on the retina of the affected eye (at the back of the eye). The use of the patch, or eye drops, is gradually tapered off, usually over the course of several months or even years, depending upon the severity of the problem.
Unfortunately, it can be difficult to diagnose vision disorders such as amblyopia in children of early age. A child may not realize, or otherwise not be able to communicate, that the child's eyesight is outside of a normal range. A child with normal vision in one eye may not notice a vision problem, even if the child's other eye is severely visually impaired. Parents are also often unable to detect that a child is suffering from a vision disorder. For example, although a child with anisometropic amblyopia may be legally blind in the affected eye, the condition often may be overlooked because the child's eyes are properly aligned, and so the child may not appear to be having vision difficulties.
There are several methods and systems that presently exist for examining a child's vision. Assessing monocular visual acuity (checking visual acuity in one eye at a time) is the best indicator of amblyopia. Monocular visual acuity assessment is the best indication of any eye pathology in children of pre-school age.
The most common method of screening vision in children of pre-school age is through the use of an eye chart. This is typically performed by positioning a wall chart across a room, at least ten feet from a child to be examined. An examiner points to optotypes (letters or symbols) that are displayed on the chart while the child covers one eye with her hand or some other type of cover. To reliably test the child's vision in one eye at a time, an adhesive patch may be used to ensure the child does not try to compensate for poor vision in one eye by “cheating” with the other eye. If the test indicates that the child can see much more clearly from one eye than the other eye, then the child may be suffering from amblyopia.
Although a vision chart can be an effective tool for measuring visual acuity, there are several disadvantages to its use for screening pre-school aged children for vision disorders. First, wall charts are not easily portable and require a special room or hallway to use. Secondly, a second examiner is usually required when using a vision chart if the child is too young to read letters or describe the appearance of the “optotype” symbols, or if the child otherwise cannot verbalize the correct response due to shyness or lack of understanding of the test. As one examiner stands at the chart and points to certain optotypes (symbols) on the chart, the second examiner holds a second chart at a closer proximity to the child. The second chart contains the same optotypes, but in a different arrangement. With one eye covered at a time, the child points to the optotypes on the second chart that correspond with the optotypes that the first examiner points to on the first chart. The second examiner is needed to monitor the child's responses while simultaneously monitoring the child to detect squinting or if the child is otherwise “cheating” on the vision exam.
A photoscreener can also be used for detecting vision disorders in very young children. A portable camera (such as the MTI PhotoScreener, by Medical Technology and Innovations, Inc., of Lancaster, Pa.) takes a photograph of a child's eyes. In the photographs, strabismus (misalignment of the eyes) and conditions that lead to amblyopia such as astigmatism, cataracts and refractive errors show up as crescents on the child's pupils. A photoscreening test is quick, noninvasive and painless, and may be useful as a high-volume initial screening method in schools. However, most photoscreeners that are presently available provide many false results (both false negatives and positives), and are not reliable for predicting visual acuity. Photoscreeners also require subsequent film processing, and may be too expensive to be used by primary care physicians or parents.
An electronic vision screener is another device that is available for performing children's vision screening. To use a vision screener (such as the Titmus Vision Screener), a child peers into a portable box and is asked to identify optotypes that are displayed on a screen in the box. Using a separate controller, an examiner can control the display of the optotypes on the screen. A vision screener can be used to test for acuity (near and far), depth perception, color perception, muscle balance (lateral and vertical phoria), and horizontal visual fields (peripheral vision of 130 degrees in each eye).
While these electronic vision screeners can be useful for screening adults' vision (and are commonly used as part of a driver's exam by local departments of motor vehicles), there are several disadvantages to using the device for screening children's vision for disorders, such as amblyopia. To use a vision screener, the child is required to recognize and identify the optotypes displayed on the screen by speaking aloud, which may not be possible for young children (as opposed to matching optotypes from an image to an optotype card). The mirrors and lighting in the device may confuse a young child, and generate less accurate results. In addition, although the device is designed to be able to separately measure the visual acuity in each eye, a young child with poor vision in one eye may shift his head to read the optotypes with the better seeing eye, and thus the amblyopic eye will not be identified. Finally, electronic vision screeners are expensive to buy and repair and are not easily portable.
Although it cannot be used to screen for vision disorders, an autorefractor (such as the SureSight™ Autorefractor by Welch Allyn, Inc.) can also be used to perform objective refraction of a child's eyes. The child is instructed to look at a light emitting from the device, while the physician adjusts the device to focus the lens at the child's eyes, one at a time. The autorefractor then measures the prescription of each eye. Generally, autorefractors are only accurate when used with children after the child receives eye drops (cycloplegia) to dilate the child's pupils and temporarily inhibit the child's strong focusing mechanism. The use of eye drops for screening purposes can become impractical, and children are generally intolerant to the delivery of eye drops for pupil dilation. In any case, an autorefractor only measures the physical characteristics of a child's eyes, and cannot indicate whether the child has a vision disorder such as amblyopia. Finally, the cost of an autorefractor renders its use prohibitive to most primary care physicians or parents.
If a child who is diagnosed with amblyopia is to be treated by “blurring” the child's stronger eye with eye drops, the child may need to be examined to determine the proper dosage of eye drops that is required to adequately “blur” the child's vision. One method of performing this examination is known as the Near Acuity Test (manufactured by PrecisionVision™, La Salle, Ill.) developed by the Pediatric Eye Disease Investigator Group (“PEDIG”). To perform this test, a physician first places eye drops in the child's stronger eye and occludes the weaker eye. The physician then uses a portable flip chart apparatus as shown in FIG. 1 to test the child's vision in the strong, but medically “blurred eye” at near distances. If the child is able to see items that are displayed on the chart, there is an indication that the child's stronger eye is not being adequately “blurred” by the eye drops to treat amblyopia.
The child is shown a series of cards 10 on the flip chart 11 of FIG. 1, one at a time, and is asked to identify what is displayed on each card. During the examination, the cards are maintained at a close distance to the child, approximately eighteen inches from the child's eye. A string 12 measuring 40 cm (0.4 meters) in length is also attached to the base of the flip chart 13. Using the string, the physician can easily measure the distance between the cards and the child's eye, and hold the flip chart at a certain distance from the child. The physician can use the string to periodically re-measure the distance between the child and the chart to maintain a constant distance during the examination.
As shown in FIG. 1, the cards each display one of the letters 14 “H”, “V”, “O”, or “T”, which are commonly-known optotypes for measuring visual acuity. The optotypes that are displayed on the cards in the flip chart are arranged in a progression of increasingly smaller sizes, which are calibrated 15 to provide an indication of visual acuity (e.g., 20/400, 20/200, 20/100, 20/50, 20/30, etc.) according to the 40 cm distance between the card and the child's eye. If the child is able to read the alphabet and is also able to see the optotype on the chart, then the child can respond by simply speaking aloud the correct letter that is displayed on each card. For children who cannot read or speak, the base of the flip chart also provides a separate display 16 of the letters “H”, “V”, “O”, and “T” in a much larger font. Instead of speaking aloud, the child can point to a letter displayed in the large text that matches the letter displayed on the card. Because the card is maintained at a distance of only 40 cm from the child's eye, the child should be able to point to and reach the base of the card. However, as it may be difficult for the child to properly point to these optotypes without moving closer to them, the child may be continually moving, it becomes difficult to maintain the 40 cm distance between the cards and the child's eyes.
Although the Near Acuity Test can assist a physician in determining the proper dosage of eye drops in the known strong eye for effectively treating amblyopia, this test cannot be used to screen for and diagnose vision disorders, such as amblyopia. The test is designed to examine a child's vision only at close distances, because it is only necessary to “blur” the child's vision at a close distance to cause the child to begin using the weaker eye. Accordingly, the test is specifically designed to test a child's “near acuity” in the strong eye at a distance of approximately 40 cm from the child's eye. The string that is attached to the base of the chart is 40 cm in length and the optotypes on the cards are calibrated and sized to be viewed at that distance. In contrast, it is necessary to test a child's vision at a longer distance to screen for amblyopia. The Near Acuity Test is intended to assess a decline of near acuity from medically inhibiting a normal eye focusing mechanism. It is not intended to screen for amblyopia. Finally, the selection of vision levels in the Near Acuity Test for providing an indication of visual acuity (e.g., 20/400, 20/200, 20/100, etc.). is not sufficient for screening vision for vision disorders such as amblyopia.
Accordingly, there is a need for a method and system for screening a child's vision to detect vision problems, such as amblyopia, that manifest in early childhood. Because children's vision is typically screened during periodic visits at a primary care doctor's office and through school vision screens, as opposed to examinations by optometrists or ophthalmologists, the method and system should be inexpensive, uncomplicated, and easily performed by a primary care practitioner, a school nurse, or even a parent. Furthermore, since children often have a short attention span and are sensitive to physical discomfort, the method and system should be quick, noninvasive, and painless.
For many of the same reasons that it is difficult to perform vision screening of pre-school aged children, challenges also exist when performing vision examinations of others, such as mentally challenged individuals or individuals suffering from any form of communication disorder, including stroke, throat trauma, or other extreme illness. Therefore, there is also a need for a method and system for quickly assessing visual acuity with a portable device in any individuals from whom subjective responses are difficult to elicit.